1. Field of the Invention
This invention relates to graphic communications, and, more particularly, to the sensing of a pen location and the transmission of graphic information sensed by a writing surface over which the pen is moved.
2. Description of the Prior Art
U.S. Pat. No. 3,767,858, by the same inventor as the present invention, discloses apparatus for converting the mechanical position of a pen into electrical signals for graphic communications purposes. The phase shifting of input signals to conductors in a grid of a writing table as a pen moves over the grid is sensed by the pen apparatus.
A phase shifting technique, using a phase locked loop, is also employed by the apparatus in U.S. Pat. No. 3,851,097, also by the same inventor as the present apparatus.
U.S. Pat. No. 3,886,311, by the same inventor as the present invention, discloses a pen and a sensor table for graphic communications which utilizes a pen capacitively coupled to an electrostatic field of the sensor table for sensing the location of the pen relative to the electrostatic field. The position of the pen is determined in terms of X and Y coordinates. The pen includes an element to detect the output of the electrostatic field and to provide an output signal in response thereto.
U.S. Pat. No. 3,904,822 describes a graphic communication system which includes a pen movable over a writing surface having a grid comprising X and Y conductors and a pickup coil in the pen inductively coupled to the X and Y conductors. The X and Y conductors cause a voltage to be generated in the pickup coil of the pen. The X and Y conductors in the table are sequentially pulsed so that the position of the pen or its pickup coil are determinable in a time relationship with respect to the pulsing of the X and Y conductors.
U.S. Pat. No. 3,983,222, the inventor of which is also the inventor of the present invention, describes another system for graphic communications in which a table includes an X and Y grid with phase displaced signals resulting from resistors in the X and Y conductors of the grid system. The grid results in an electrostatic field which is sensed by a pickup element in the pen as it moves over the grid. Since the phase shifting is in a predetermined manner, the signal detected by the pen results in a correlation between the location of the signal sensed by the pen and the electrostatic field generated by the X and Y conductors in accordance with the known phase and frequency of the signal from the X and Y conductors at a particular time.
U.S. Pat. No. 4,022,971, also by the same inventor as the present invention, discloses another graphic communication system also having X and Y conductors which generate an electric field and the field is sensed by the pen through a phase shifting system which differs from the phase shifting system of the patent described in the preceding paragraph. In the U.S. Pat. No. 4,022,971 patent, the X and Y conductor grid includes a central zero degree conductor, with conductors on either side of the zero degree conductor being displaced in phase. The dynamic range of the surface is limited to small incremental areas by multiplexing circuitry.